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Stellar evolution
The theory of '''stellar evolution '''is simply how a star changes during its lifetime. The length of its life depends on its mass; the least massive stars last trillions of years, which is considerably longer than the current age of the universe and the most massive ones last only a few million years. All stars are born in nebulae or molecular clouds. The gas at the center heats up and drags material to it. Then everything explodes and a star is born. Small stars, like Megaladon, Sol and Alpha Centauri A live for about 10 billion years and grow into a red giant. The red giant survives for approximately 1 billion years and is expelled in a planetary nebula. Meanwhile the core of the dead star cools down and stays as a white dwarf for hundreds of billions of years. The smallest stars live for about a trillion years and shrink into a white dwarf. Brown dwarfs shine for short periods of time and fade out while the most massive stars heat up inside their cores, expand as a red supergiant or hypergiant, and soon, the gravity of the star's core causes it to collapse inward, and causing an explosion called either a supernova or hypernova. The squashed core is extremely massive (2-5 kmp) and either turns into a neutron star or a black hole. A neutron star is a dense, hot and extremely small body around 20 km across with very strong gravity. However, neutron stars can spin. A spinning neutron star is known as a pulsar. A neutron star can have mass between 2,000 and 4,000 mp. If it reaches 4,000 mp, the neutron star shrinks even further, into a tiny, dense object with gravity so strong that light cannot escape from it. This is known as a black hole. Hypergiants, such as VY Canis Majoris explode in a hypernova and collapse into a black hole. The supermassive black hole at the Core of the Galaxy is probably left over from the explosion of a hypergiant. Evolutionary scenarios Brown dwarfs Brown dwarfs are tiny, gas giant sized stellar objects with very low mass. The core of a brown dwarf is still hot, but not hot enough to fuse hydrogen into helium. They do not radiate much energy into space and therefore stay near-invisible. The brown dwarf is not hot enough to expand into a red giant and burn out at the end of its life. Low-mass stars What happens after a low-mass star ceases to produce energy through fusion is not directly known: the universe is thought to be around 13.7 billion years old, which is less time (by several orders of magnitude, in some cases) than it takes for the fusion to cease in such stars. Some stars may fuse helium in core hot-spots, causing an unstable and uneven reaction as well as a heavy stellar wind. In this case, the star will form no planetary nebula but simply evaporate, leaving little more than a small brown dwarf. A star of less than about 500 mass points. will never be able to fuse helium even after the core ceases hydrogen fusion. There simply is not a stellar envelope massive enough to exert enough pressure on the core. These are class M red dwarfs, such as Proxima Centauri, some of which will live thousands of times longer than the Sun. Recent astrophysical models suggest that red dwarfs of 0.1 solar mass may stay on the main sequence for some six to twelve trillion years, and take several hundred billion more to slowly collapse into a white dwarf. If a star's core becomes stagnant, it will still be surrounded by layers of hydrogen which the star may subsequently draw upon. However, if the star is fully convective (as thought to be the case for stars less than 0.25 solar masses) it will not have such surrounding layers. If it does, it will develop into a red giant as described for mid-sized stars below, but never fuse helium as they do; otherwise, it will simply contract until electron degeneracy pressure halts its collapse, becoming first a blue dwarf and then a white dwarf. Mid-mass stars Stars of mass between 800 and 10,000 mass points live for around 10 billion years. These stars are born in massive molecular clouds and grow in size and luminosity over a period of a billion years. Some examples of middle-aged stars include Megaladon, Sol and Alpha Centauri A. These stars are about 4,6 billion years old and are yellow dwarfs. These stars live for about 10 billion years, and die as white dwarfs.